The polymerization of monomers such as vinyl compounds or (meth)acrylates usually involves a considerable volume contraction, which is also referred to as polymerization shrinkage. This polymerization shrinkage of the monomers used can lead for instance in the case of dental materials inter alia to disadvantageous shrinkage stresses and to the formation of marginal gaps in the case of filling composites, to reduced substrate adhesion in the case of fixing composites or coating materials as well as to the impairment of the dimensional stability of prosthesis plastics. Accordingly, low-shrinkage monomers have attracted much interest in the dental field (cf. N. Moszner, U. Salz, Progress Polymer Sci. 26 (2001) 535-576 and N. Moszner, U. Salz, Macromol. Mater. Eng. 292 (2007) 245-271).
Relatively low-shrinkage monomers used in the dental field are in particular the higher molecular weight dimethacrylate cross-linkers Bis-GMA and UDMA, which exhibit a polymerization shrinkage ΔVp of 6.0 and 6.1 vol.-%, respectively. However, these cross-linkers have a very high viscosity (Bis-GMA: η=800-1000 Pa·s; UDMA: η=10 Pa·s), so that they usually have to be used in mixture with low-viscosity dimethacrylate diluents such as triethylene glycol dimethacrylate, which however exhibit a considerably higher polymerization shrinkage (triethylene glycol dimethacrylate: ΔVp=14.5 vol.-%).
Radically polymerizable cyclic monomers are generally characterized by a lower polymerization shrinkage as compared to linear monomers (cf. R. K. Sadhir, R. M. Luck, Expanding Monomers, CRC Press, Boca Raton etc. 1992). However, these monomers often have a considerably reduced reactivity as compared to linear monomers, whereby their practical applicability particularly in the dental field is limited considerably.
The term dimer acids generally refers to cyclic, particularly polyvalent, carboxylic acids which can be obtained by cyclodimerization of unsaturated fatty acids. Typically, these are cyclic di- or tricarboxylic acids that are obtainable for instance by alumina-catalyzed dimerization of unsaturated fatty acids, such as oleic acid, linoleic acid or tall oil. Such dimer acids usually have 36 carbon atoms on average (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A8, VCH, Weinheim and New York 1987, pp. 535-536). High-purity products are commercially available, which can contain for example the following unsaturated dimer acids:

By hydrogenation of unsaturated dimer acids, the corresponding hydrogenated dimer acids are obtainable. For example, the following hydrogenated dimer acids can be obtained by hydrogenation of the unsaturated dimer acids shown above:

US 2008/0318188 A, which is hereby incorporated by reference, describes dimethacrylates derived from dimer acids and their use in dental compositions. These exhibit low shrinkage during polymerization, but produce polymer networks with high flexibility and low elastic modulus, which is disadvantageous in particular for use in dental materials such as filling composites (cf. M. Trujillo-Lemon, J. Ge, H. Lu, J. Tanaka, J. W. Stansbury, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 3921-3929).